1. Field of the Invention
This invention relates to microwave amplifiers, and more particularly, to traveling wave tubes of the type wherein a plurality of electron beam modulating delay circuits are contained in an envelope with an electron gun and associated beam deflecting circuitry.
2. Prior Art
It is well known that communication satellites are severely constrained by limitations on maximum size, maximum weight, and maximum available power. These three factors are directly interrelated with one another and with launch cost; the major initial capital investment factor. Efficient use of available satellite power, therefore, is essential to achieving adequate satellite communications capacity at reasonable initial investment.
The more recent designs for communications satellites generally provide a substantial number of electromagnetic energy beams, each of which beams utilizes at least one traveling wave tube microwave amplifier. It is a problem with traveling wave tubes of a type which have heretofore been employed in satellite communications systems, that their characteristics of operation render them wasteful of electrical energy, especially in the idle channel mode. Each traveling wave tube (TWT) contains a relatively inefficient electron beam source which requires very high anode potentials to control the velocity of an electron stream. The basic components and operation of a TWT are explained in a work entitled "Traveling Wave Tubes", by J. R. Pierce (1950).
During such times as the associated channel is not transmitting information, the TWT does not amplify RF signals but will nevertheless draw a substantial share of prime power. In systems operated in a time division multiple access (TDMA) mode, an idle TWT will draw as much as 60 to 70 percent of the power required during continuous RF modulation. Semiconductor switches which, are relatively lossless, can switch anode potentials in the order of several thousand volts, and yet be space qualified for an orbit life of approximately ten years, are not available under present technology.
The current practice under TDMA technology, as applied to communications satellites, is to employ lossy semiconductor switches to transfer an RF signal to individual TWTs associated with respective antenna ports. This practice, of course, does not alleviate the standby power consumption problem associated with TWTs. It would be preferable, therefore, to provide switching at the TWT output. However, semiconductor diode switches cannot reliably switch the high RF power output of the TWTs, and therefore do not satisfy the lifetime requirements of the space environment. Ferrite-type switches can generally switch high RF power levels, but are too slow for TDMA applications. The problem remaining in the prior art, therefore, is to provide an energy efficient microwave amplifier which can operate at high switching speeds in the space environment.